Recently, from a view point of the high energy price of crude oil or the like and the exhaustion of oil resources due to the increase in energy consumption volume on a global scale, there has been observed the vigorous energy resource development with respect to oil fields having a large depth (deep layer oil fields) which had not been noticed, oil fields and gas fields in a severely corrosive environment which are in a so-called “sour” environment containing hydrogen sulfide or the like, and oil fields and gas fields in a far north region which is in a severe weather environment. Steel tubes for oil wells used in these oil fields and gas fields are required to have high strength, excellent corrosion resistance (sour resistance) and excellent low-temperature toughness.
Conventionally, in oil fields and gas fields in an environment containing carbon dioxide gas CO2, chloride ions Cl− and the like, as an oil well tube used to drill, 13% Cr martensitic stainless steel tube has been popularly used. Recently, the use of improved version 13Cr martensitic stainless steel having a chemical composition, wherein the content of C is decreased and the contents of Ni, Mo and the like are increased, has been spreading.
For example, Japanese Patent Application Laid-open No. 10-1755 discloses a method of manufacturing a martensitic stainless steel (steel plate) wherein the corrosion resistance of 13% Cr martensitic stainless steel has been improved. The martensitic stainless steel disclosed in Japanese Patent Application Laid-open No. 10-1755 is manufactured by hot working a steel having a chemical composition containing by weight %, 10 to 15% Cr, 0.005 to 0.05% C, 4.0 to 9.0% Ni, 0.5 to 3% Cu, and 1.0 to 3% Mo, wherein the Ni equivalent amount is adjusted to −10 or more, followed by air-cooling to a room temperature, thereafter, heat treatment at a temperature equal to or above an Ac1 point at which an austenite fraction becomes 80% or less and, further, heat treatment at a temperature at which the austenite fraction becomes 60% or less. The thus manufactured martensitic stainless steel has a microstructure constituted of tempered martensitic phase, martensitic phase and retained austenitic phase, wherein the total fraction of tempered martensitic phase and martensitic phase becomes 60 to 90%. It is described in Japanese Patent Application Laid-open No. 10-1755 that the martensitic stainless steel enables corrosion resistance and sulfide stress corrosion cracking resistance in a wet carbon dioxide environment and a wet hydrogen sulfide environment to be improved.
Japanese Patent No. 5109222 (Japanese Patent Application Laid-open No. 2005-336595) discloses a method of manufacturing a high-strength stainless steel tube for oil wells having excellent corrosion resistance. The high-strength stainless steel tube disclosed in Japanese Patent No. 5109222 (Japanese Patent Application Laid-open No. 2005-336595) is manufactured by heating a steel having a chemical composition containing by mass %, 0.005 to 0.05% C, 0.05 to 0.5% Si, 0.2 to 1.8% Mn, 0.03% or less P, 0.005% or less S, 15.5 to 18% Cr, 1.5 to 5% Ni, 1 to 3.5% Mo, 0.02 to 0.2% V, 0.01 to 0.15% N, 0.006% or less o, wherein Cr+0.65Ni+0.6Mo+0.55Cu−20C≥19.5 and Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≥11.5 are satisfied, followed by hot working into a seamless steel tube, cooling to a room temperature at a cooling rate equal to or above a cooling rate of air-cooling, reheating to a temperature of 850° C. or more, cooling down to a temperature equal to 100° C. or below at a cooling rate of air-cooling or more and, thereafter, quenching-tempering treatment where the seamless steel tube is heated to 700° C. or below. The high-strength stainless steel tube has a microstructure containing 10 to 60% of ferrite phase by a volume fraction and the balance being martensitic phase, and a yield strength of 654 MPa or more. It is described in Japanese Patent No. 5109222 (Japanese Patent Application Laid-open No. 2005-336595) that the high-strength stainless steel tube for oil wells has high strength, sufficient corrosion resistance also in a high temperature severe corrosion environment up to a temperature of 230° C. containing CO2 and chloride ions Cl− and, further, high toughness with an absorbed energy of 50 J or more at a temperature of −40° C. in a Charpy impact test.
As a seamless steel tube for oil wells, it is necessary for the steel tube to have various wall thicknesses and diameters. In the manufacture of a heavy-walled seamless steel tube, when the steel tube is manufactured using conventional hot working, along with the increase in wall thickness of the steel tube, it is difficult to impart desired processing strain to the wall thickness center portion of the steel tube. Hence, there is a tendency for the microstructure of the wall thickness center portion of the steel tube to become coarse. Accordingly, the toughness of the wall thickness center portion of the heavy-walled steel tube is liable to be deteriorated compared to the toughness of the wall thickness center portion of a thin-walled steel tube. Japanese Patent Application Laid-open No. 10-1755 and Japanese Patent No. 5109222 (Japanese Patent Application Laid-open No. 2005-336595) aim at the application thereof to a steel tube having a wall thickness of 12.7 mm at maximum. Neither Japanese Patent Application Laid-open No. 10-1755 nor Japanese Patent No. 5109222 (Japanese Patent Application Laid-open No. 2005-336595) refers to the improvement of low-temperature toughness of heavy-walled seamless steel tube having a wall thickness exceeding 12.7 mm.
It could therefore be helpful to provide an equipment line for manufacturing a seamless steel tube which can manufacture a heavy-walled stainless seamless steel tube having excellent low-temperature toughness at a low cost. Further, it could be helpful to provide a method of manufacturing a high-strength heavy-walled stainless seamless steel tube for oil wells having a yield strength exceeding 654 MPa, excellent corrosion resistance in a hot corrosive environment and excellent low-temperature toughness at the wall thickness center portion thereof by making use of the equipment line. In this specification, “a heavy-walled seamless steel tube” means a seamless steel tube having a wall thickness exceeding 13 mm and equal to 100 mm or less.